Set-up testing of the mongo DB client

[kriswest]

Is your feature request related to a problem? Please describe.
One or more recent PRs were merged with errors in the mongoDB client implementation, which is currently not used when testing and has little test coverage (31.46% vs 91.41% for the file DB client, see https://app.codecov.io/gh/finos/git-proxy/tree/main/src%2Fdb)

Describe the solution you'd like
At least the DB tests are repeated in CI against the mongo DB client. That will require a mongoDB instance to be stood up during the test (that can't be mocked and produce a valid result AFAIK).

Additional context
I note the CI job does start a Mongo DB instance, but it is not then used when testing... See:

git-proxy/.github/workflows/ci.yml

Lines 39 to 42 in 4bf2276

	- name: Start MongoDB
	uses: supercharge/mongodb-github-action@90004df786821b6308fb02299e5835d0dae05d0d # 1.12.0
	with:
	mongodb-version: ${{ matrix.mongodb-version }}

[coopernetes]

Related: #1165